1. Field of the Invention
The present invention relates to an image forming apparatus (laser beam printer or the like), and more particularly to a technique for correcting a curl of a transfer material (paper or the like).
2. Description of the Background Art
In an image forming apparatus (a laser beam printer or the like), a sheet of paper passes through a fixing part and a toner image transferred to the sheet of paper is thereby fixed onto the sheet of paper. At that time, the sheet of paper is heated and pressed by the fixing part and a curl thereby appears in the sheet of paper.
In order to avoid such a problem, there is a technique for correcting the curl appearing in the sheet of paper by pressing the sheet of paper against a guide member. In an image forming apparatus disclosed in Japanese Patent Application Laid Open Gazette No. 2001-48399 (Patent Document 1), for example, a first conveyance roller is provided on the downstream side of the fixing part and a second conveyance roller is provided on the further downstream side thereof. In a conveyance path leading from the first conveyance roller to the second conveyance roller, provided is a guide member which is curved in a direction opposite to a direction of a curl of a sheet of paper. The conveyance speed at which the second conveyance roller conveys the sheet of paper is set to a value larger than that of the conveyance speed at which at which the first conveyance roller conveys the sheet of paper. The sheet of paper is conveyed while being pressed against the guide member which is curved in a direction opposite to a direction of the curl of the sheet of paper.
Some of the image forming apparatuses comprise, as a printing mode, not only a single-sided printing mode where a single side of a sheet of paper is printed but also a double-sided printing mode where both sides (first and second sides) of a sheet of paper are printed.
In the single-sided printing mode, the curl of the sheet of paper can be corrected with the application of the above correcting technique.
In the double-sided printing mode, however, the curl of the sheet of paper cannot be appropriately corrected simply with the application of the above correcting technique. Specifically, both in printing of a first surface and in printing of a second surface, if the sheet of paper is pressed against the guide member with the same pressing force as that in a single-sided printing, the curl of the sheet of paper cannot be appropriately corrected. This phenomenon results from the fact that the curvature of a curl appearing in the sheet of paper in the printing of the second surface is different from that of a curl appearing in the sheet of paper in the printing of the first surface, as discussed later.